The invention relates to computed tomography (CT) and, more particularly, to an apparatus and a method for use in a volumetric CT scanning system for reconstructing images.
Computed tomography (CT) is a technique that generally involves subjecting a patient to x-rays, acquiring digital x-ray projection data of a portion of the patient's body, and processing and back-projecting the digital x-ray projection data to produce an image that is then displayed on a display monitor of the CT system. CT systems typically comprise a gantry, a table, an x-ray tube, an x-ray detector array, a computer and a display monitor. The computer sends commands to controllers of the gantry to cause the controllers to rotate the x-ray tube and/or the detector array at a particular rotational speed. The detector array is usually comprised of either a curved array (third generation CT system) of detector elements or a ring (fourth generation CT system) of detector elements. In the case where a ring of detector elements is used, only the x-ray tube is rotated.
In third and fourth generation CT systems, relative rotational motion is produced between the detector array and the x-ray tube about the patient's body. As this relative rotational motion is produced, the computer controls the data acquisition process performed by the x-ray tube and the detector array to acquire digital x-ray radiographs. The computer then processes and back-projects the digital x-ray radiograph data by performing a reconstruction algorithm and displays the reconstructed CT image on the display monitor.
In medical screening applications, dose delivered to the patient is always of great concern. To minimize risk to the patient, the dose delivery by diagnostic imaging procedures is minimized. This is especially true when the diagnostic procedures are utilized for screening applications. In general, lower dose, lower resolution CT data are acquired in the initial screening examination. However, if some pathology is observed, such as a solitary lung nodule observed during a thoracic imaging procedure, the patient is normally required to return to the hospital for an additional high-dose, high-resolution CT procedure. The administered radiation places the patient at higher risk and reduces the productivity of the hospital since the patient is required to make a follow-up visit.
In CT systems that utilize area detectors, the CT scanning procedure is known as volumetric CT scanning because CT data i; acquired by the area detector for a volume of the patient. By using high resolution area detectors in a volumetric CT scanning system, it is possible to reconstruct spatial volume data at extremely high resolution. In some cases, the resolution may be an order of magnitude higher than the resolution obtained with other types of detectors. Volumetric CT scanning enables diagnostic procedures to be performed quicker than with other types of CT scanning, which facilitates patient throughput for the hospital.
However, manipulating data at this higher resolution is generally prohibitive due to the long reconstruction times and the physical size of the reconstructed volume. High resolution area detectors have such a large number of pixels that the amount of data collected is extremely large, which presents problems in terms of storing, retrieving and manipulating the data in real time. It would be beneficial if a volumetric CT scanning system could be used to generate a relatively low resolution volumetric image that could be processed to identify potentially problematic areas. It would also be beneficial if a high resolution image of the area identified as being potentially problematic could be reconstructed. In this manner, the benefits of using a high resolution area detector could be realized, but not at the expense of reconstruction time or data storage/manipulation. Furthermore, if all of the data needed to generate the high and low resolution images could be acquired during a single scan of the patient, the patient would not be required to revisit the hospital, which could improve the productivity of the clinical environment. In addition, the radiation dose that is administered to the patient would be reduced, thus reducing the risk to the patient.
Accordingly, a need exists for a method and apparatus for use in a volumetric CT scanning system that reduces the amount of data that is used in initially reconstructing the image, but which utilizes all of the acquired data associated with a particular region of interest to retrospectively reconstruct a high resolution image of the region of interest. The initially reconstructed image would contain sufficient contrast information to enable certain problematic areas to be identified. The retrospectively reconstructed high resolution image of the problematic area could then be processed and analyzed to further evaluate the problematic area.